1. Field of the Invention
The present invention relates to a developing device for developing an electrostatic latent image into a visible image in an electrophotographic apparatus and electrostatic recording apparatus and, in particular, a developing device for obtaining a high-quality image using a one-component developing agent.
2. Description of the Related Art
Recently, a developing system using a one-component developing agent has been used in this field of art.
This developing system, differing from a developing system using a two-component (toner/carrier) type developing agent, obviates the necessity of providing a developing agent carrier, magnet roll, toner density control apparatus, etc. It is, therefore, possible to provide a compact, low-cost developing system.
The impression development method has been known as one of the developing methods using a one-component developing agent.
This method has the feature in that a toner carrier having a thin toner (developing agent) layer is brought into contact with an electrostatic latent image on a photosensitive drum at a substantially zero relative peripheral speed as disclosed in U.S. Pat Nos. 3,152,012 and 3,731,146 (IBM) and Published Unexamined Japanese Patent Applications 47-13088 and 47-13089.
The disclosed method ensures a simple and compact device and allows the use of color toner.
In the compression development method, the toner carrier is pressed against, or brought into contact with, a latent image on the photosensitive drum to effect development. It is, therefore, necessary to use a developing roller of proper elasticity and electroconductivity as the toner carrier. It is essential that, if an electrostatic image carrier is made up of a rigid structure, a developing roller be made up of an elastic body so as to prevent a possible damage to the image carrier.
U.S. Pat. Nos. 3,866,574 and 3,893,418, for example, disclose a non-contacting developing method as another one-component developing method.
This method comprises applying an AC electric field across a gap between a toner (developing agent) and an electrostatic latent image, on a photosensitive drum which is located near the toner side, to allow the toner to fly onto the latent image for development. It has been usual practice to form the developing roller as a rigid body.
It has generally been known that the use of the one-component developing method presents an image defect called a "sleeve ghost".
The generation of the sleeve ghost is ascribed to the state of formation of a toner layer on the developing roller. Stated in more detail, if, in a present development cycle, a toner-deposited area on the electrostatic latent image side is recovered under the same condition as in a previous development cycle in terms of a toner layer thickness, amount of charge-, etc., a toner density difference is produced between the two due to their "memory" difference.
The solutions to this problem are as set out below:
(1) a scraping member, such as a blade, is placed in contact with the developing roller to allow all the remaining toner to be removed from the developing roller at the completion of development. By so doing it is possible to eliminate the "memory" difference.
(2) A remaining deposited-toner is all removed, by a sponge and brush, from the developing roller at the completion of development to eliminate its "memory" difference.
(3) A bias voltage is applied to, for example, a conductive roller, moving the latter nearer the developing roller to electrostatically remove all the remaining toner left without being spent by development from the developing roller. In this way it is possible to eliminate the "memory" difference.
Either approach to this problem is to remove all the remaining toner from the developing roller at a time after a toner has been developed. By so doing, the "memory" difference on the developed pattern is removed. The aforementioned approach is reasonable from the standpoint of removing the cause involved.
However, either solution involves some problem.
In the solution (1), the developing roller is liable to be damaged by the blade and the material used, as well as the condition set, is restricted to an appreciable extent. It is also necessary to provide a means for conveying the once-removed toner again to a toner supply section. As a result, the arrangement becomes complex.
In the solution (2), the problem as pointed out by the approach (1) can be solved to some extent. Further, a stronger rubbing force is required to adequately remove the remaining toner by the means as set out below. An excessive burden is inflicted onto the associated device. This causes a variation of a rotational speed, etc., of the developing roller and a striped image defect is liable to be produced.
In the solution (3), a new electroconductive roller, bias power source, etc., are required as extra component parts and, in addition, the disadvantage as set out in the approach (3) remains to be solved.